Skin-whitening activity of a tranexamate has been known, and the same has been used as a skin-whitening ingredient in compositions for external use, such as drugs, quasi-drugs, and cosmetics. For example, Japanese Patent Laid-Open No. 04-46144 (Patent Document 1) discloses an anti-pigmentation agent for external use using a tranexamate as an active ingredient. Japanese Patent Laid-Open No. 2003-306419 (Patent Document 2) exemplifies tranexamic acid and derivatives thereof as a skin-whitening ingredient to be usable with Coenzyme Q10. Japanese Patent Laid-Open No. 2004-107262 (Patent Document 3) exemplifies cetyl tranexamate as an oil-soluble skin-whitening ingredient to be usable with an L-ascorbic acid tetra-branched fatty acid ester derivative.
However, there has been a problem that a tranexamate is sparingly-soluble in water or oil, and stable mixture thereof in a composition over an extended time period is difficult, and improvement of the solubility and the stability of the formulation have remained unsolved for successful formulation.
Japanese Patent Laid-Open No. 2002-234836 (Patent Document 4) discloses an anti-stress external preparation for skin using a tranexamate as an active ingredient. The same discloses a method to dissolve a tranexamate using a large amount of an oil component such as an olive oil. However, by this method, the formulation form is limited to a cream or a milky lotion due to the use of a large amount of an oil component. Additionally, the obtained external preparation is sticky and oily and the sense of use is unfavorable.
Further, Japanese Patent Laid-Open No. 2006-306744 (Patent Document 5) discloses an external preparation for skin using a tranexamate or a salt thereof as well as a silicone oil. The same teaches that by the combination of a tranexamate or a salt thereof and a silicone oil the solubility of the tranexamate or a salt thereof in the composition can be enhanced, and the activity of the tranexamate can last over an extended time period. Although a silicone oil is known as an oil component giving less heavy sense of use, due to the necessity of the use of a large amount of the same to dissolve the tranexamate, the range of choice for the formulation forms of the external preparation for skin becomes narrow.